在BroadcastReceiver，當我們建立一個AlertDialog並show出來的時候，出現了下面的錯誤：

12-24 14:10:57.025: E/AndroidRuntime(17600): java.lang.RuntimeException: Unable to start receiver com.ustc.broadcastreceiver.ForceOfflineReceiver: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not for an application

下面的方法可以解決：

第一步：

在呼叫show方法之前，設定下面的屬性：

dialog.getWindow()
				.setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);

12-24 14:15:22.793: E/AndroidRuntime(17979): java.lang.RuntimeException: Unable to start receiver com.ustc.broadcastreceiver.ForceOfflineReceiver: android.view.WindowManager$BadTokenException: Unable to add window

這個一看就知道了，許可權拒絕，需要新增許可權：

第二步：

在清單檔案中新增許可權：

 <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>

package com.ustc.broadcastreceiver;

import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.content.Intent;
import android.view.WindowManager;

public class ForceOfflineReceiver extends BroadcastReceiver {

	@Override
	public void onReceive(final Context context, Intent intent) {

		AlertDialog.Builder builder = new Builder(context);
		builder.setTitle("提示");
		builder.setMessage("強制下線");
		builder.setCancelable(false);
		builder.setPositiveButton("ok", new OnClickListener() {

			@Override
			public void onClick(DialogInterface dialog, int which) {

				ActivityCollector.finishAll();
				Intent intent = new Intent(context, LoginActivity.class);
				intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
				context.startActivity(intent);

			}
		});
		AlertDialog dialog = builder.create();

		dialog.getWindow()
				.setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);

		dialog.show();
	}

}